WASHINGTON — Poor Defense Department oversight of private companies that fly military missions in war zones was partially responsible for a 2004 crash that killed six people in Afghanistan, federal accident investigators said Monday.

The National Transportation Safety Board recommended that the Defense Department coordinate with the Federal Aviation Administration to better monitor the charter flights increasingly being used to carry small numbers of troops and cargo in Iraq and Afghanistan.

An NTSB statement said the agency is "concerned that the unique risks presented by operations in remote overseas locations have not been adequately addressed for civilian contractors that provide air transportation services to the U.S. military."

The flight was operated by Presidential Airways of Melbourne, Fla., a part of the same company as Blackwater USA, a private security firm assisting private companies and the military in war zones.

Pilots on the CASA 212 twin-turboprop were flying over a remote mountainous region of Afghanistan on Nov. 27, 2004, when they entered a tight canyon ringed by peaks as high as 16,580 feet. They tried to climb over the mountains, but crashed at 14,650 feet, the NTSB said.

The pilots had not used oxygen, as is required when flying over 10,000 feet for prolonged periods, the NTSB said. Lack of oxygen at high altitudes can interfere with pilots' mental skills. The NTSB also noted the company's failure to require crews to fly specific routes and follow safety regulations.

For example, the military requires planes it charters to be equipped with a device that allows people on the ground to follow the flight's route. But the NTSB said it found no evidence that the military had tried to enforce the rule.

Without the device, searchers had difficulty locating the wreckage. Searchers did not find it for nearly 24 hours. The search was also slowed because the company did not discover that the plane was missing until nearly six hours after the crash.

Lt. Col. Todd Vician, a Defense Department spokesman, said the Pentagon will review the recommendations.

Mark Corallo, a spokesman for Blackwater, said that the NTSB report "fails to understand the realities of operations in a war zone."